linux-kernel - Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer

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Date:	Fri, 12 Sep 2014 11:59:29 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Marc Zyngier <marc.zyngier@....com>,
	Christopher Covington <cov@...eaurora.org>
CC:	Doug Anderson <dianders@...omium.org>,
	Will Deacon <Will.Deacon@....com>,
	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to
 specify the physical timer

On 09/12/14 05:14, Marc Zyngier wrote:
> Hi Christopher,
>
> On 12/09/14 12:43, Christopher Covington wrote:
>> Hi Marc,
>>
>> On 09/11/2014 01:43 PM, Marc Zyngier wrote:
>>> On 11/09/14 18:29, Doug Anderson wrote:
>>>
>>>> I did this in the past (again, see Sonny's thread), but didn't
>>>> consider myself knowledgeable to know if that was truly a good test:
>>>>
>>>>        asm volatile("mrc p15, 0, %0, c1, c1, 0" : "=r" (val));
>>>>        pr_info("DOUG: val is %#010x", val);
>>>>        val |= (1 << 2);
>>>>        asm volatile("mcr p15, 0, %0, c1, c1, 0" : : "r" (val));
>>>>        val = 0xffffffff;
>>>>        asm volatile("mrc p15, 0, %0, c1, c1, 0" : "=r" (val));
>>>>        pr_info("DOUG: val is %#010x", val);
>>>>
>>>> The idea being that if you can make modifications to the SCR register
>>>> (and see your changes take effect) then you must be in secure mode.
>>>> In my case the first printout was 0x0 and the second was 0x4.
>>> The main issue is when you're *not* in secure mode. It is likely that
>>> this will explode badly. This is why I suggested something that is set
>>> by the bootloader (after all. it knows which mode it is booted in), and
>>> that the timer driver can use when the CPU comes up.
>> What exactly does "exploding badly" look like? Causing and undefined
>> instruction exception? That's just a branch with a mode switch. Any reason the
>> code couldn't deal with that or even use that to its advantage?
> We surely can handle the UNDEF and do something there. We just can't do
> it the way Doug described it above.
>
>

I suggested doing that for something else a while ago and Will and Dave
we're not thrilled[1]. The suggestion back then was to use DT to
indicate what mode the kernel is running in.

[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2012-June/105321.html

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